我如何“添加别名”当我不控制源时枚举值？ [英] How can I "add aliases" to enum values when I don't control the source?

问题描述

（这一个的后续问题）

我'编写一个程序，使用一个库，其头文件定义

 枚举foo：unsigned {first_foo，second_foo}; 
  

现在，我想添加一些别名到 foo 值。如果我是图书馆的来源，我会写：

 枚举foo：unsigned {
 first_foo，
 second_foo，
 best_foo = first_foo，
 worst_foo = second_foo，
 oldest_foo = first_foo，
 newest_foo = second_foo; 
}; 
  

...但我不控制源。所以，我想写：

 枚举吧：foo {
 best_foo = first_foo，
 worst_foo = second_foo，
 oldest_foo = first_foo，
 newest_foo = second_foo; 
}; 
  

但这是无效的C ++，因为foo不是一个整体类型。如果我试图解决它，并使用底层类型：

 枚举栏：std ::底层类型< foo> {/ *等等* /} 
  

它编译，但是 - 我不会值类型为foo，并且我收到关于 bar 和 foo 之间的比较的编译器警告。

我可以使用 static const 变量：

  static const auto best_foo = first_foo; 
 static const auto worst_foo = second_foo，
 static const auto oldest_foo = first_foo，
 static const auto newest_foo = second_foo; 
  

但我不想冒险进入文本部分或符号表。 / p>

那么我该怎么办？

解决方案

code>枚举栏完全分开，然后重载一个$ $ $ $ $ $ $ $ $ $ $ $ $ $ $

  constexpr foo operator +（bar const input）
 {
 return static_cast< foo> （输入）; 
} 
 
 constexpr foo operator +（foo const input）
 {
 return input; 
} 
  

(Followup question to this one)

I'm writing a program that uses a library, whose header file defines

enum foo : unsigned { first_foo, second_foo };

Now, I want to add some aliases to foo values. If I were in control of the library's source, I would write:

enum foo : unsigned { 
    first_foo,
    second_foo,
    best_foo   = first_foo,
    worst_foo  = second_foo,
    oldest_foo = first_foo,
    newest_foo = second_foo; 
};

... but I don't control the source. So, I would have liked to write:

enum bar : foo { 
    best_foo   = first_foo,
    worst_foo  = second_foo,
    oldest_foo = first_foo,
    newest_foo = second_foo; 
};

but that is invalid C++, since foo is not an integral type. If I try to work around it and use the underlying type:

enum bar : std::underlying_type<foo> {  /* etc. etc. */ }

it compiles, but then - I don't get values of type foo, and I get compiler warnings about comparison between bar's and foo's.

I can use static const variables:

    static const auto best_foo    = first_foo;
    static const auto worst_foo   = second_foo,
    static const auto oldest_foo  = first_foo,
    static const auto newest_foo  = second_foo; 

but I don't want to risk them going into the text section, or the symbol table.

So, what should I do?

解决方案

You can define enum bar completely separately, then overload the unary + operators for both bar and foo to return foo:

constexpr foo operator + ( bar const input )
{
    return static_cast <foo> ( input );
}

constexpr foo operator + ( foo const input )
{
    return input;
}

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